Modern cars comprise electronic control systems such as anti-lock-braking systems (ABS), dynamic stability systems, anti-spin systems and traction control systems. Besides these active control systems there also exist driver safety information systems as road friction indicators and tire pressure monitoring systems which present to the driver information about driving and vehicle conditions. Indirect tire pressure monitoring systems are based on indirect detection values of a modern vehicle, such as the wheel speed signals, in order to obtain pressure estimations of a tire. Theses pressure estimations are typically compared with calibration values in order to determine specific tire pressure situations, such as a tire puncture. The calibration values are usually calculated as averages of data gathered during an initial calibration period.
EP 1 403 100 A1 discloses a calibration routine for calculating such calibration values. After having collected a number of data, a first type calibration value is determined which is used to compare it with further collected data in order to determine a tire pressure drop. After having determined the first type calibration value this value is further fine-tuned to obtain a second type calibration value by collecting further data during a fine tuning period.
During the fine tuning period, the calibration value is monitored for tire growth. When tire growth is detected during the fine tuning period the calibration process is restarted. However, no such tire growth is detected before the first type calibration value has been determined.
WO 03/086789 discloses a similar process wherein during the fine tuning period a tire growth situation is differentiated from a pressure loss situation in order to reliably detect the tire growth situation. Again, no tire growth or pressure loss situation will be detected until a first learning value (calibration value) has been determined.